1. Field of the Invention
The present invention relates to a drive belt.
2. Description of the Related Art
Drive belts of the present type are generally known through their application in continuously variable transmissions intended for the transmission of mechanical power at a continuously variable speed and torque ratio between an engine and a load in particular for automotive purposes. Such transmission and drive belt are, for instance, described in the European patent application EP-A-1219860.
The known drive belt generally comprises an endless carrier and an array of several hundred, correspondingly shaped transverse elements, each having a longitudinally facing front and rear main surfaces, which elements are essentially oriented in parallel in the drive belt such that the front main face of the elements each face the rear main face of an respectively adjacent element and vice versa. The carrier is provided in a slot of the elements such that the elements may freely slide over the carrier in the direction of movement, i.e. longitudinal direction thereof.
The belt's transverse elements are each provided with a so-called rocking edge on the front main face, which rocking edge forms the transition between a bottom section of the element having a longitudinal dimension or thickness that reduces in radially inward direction and a top section of the element having a thickness that is more or less constant, at least in comparison. The transverse elements each further include an at least generally trapezoid-shaped lower or body part that is located below the carrier receiving slot, an at least generally arrow-head shaped upper or head part and a central or pillar part placed between and interconnecting said head and body parts. Usually, the said rocking edge is provided in the said body part, i.e. radially inward form the carrier receiving slot, which body part is further provided with laterally facing side or contact surfaces, one on either side thereof, for arriving into a frictional contact with pulleys of the transmission during operation thereof.
In the continuously variable transmission the drive belt rotationally connects two pulleys, each having two essentially conically shaped pulley discs that define a V-groove of variable width, where between a longitudinally curved section of the drive belt is located. The posture of the drive belt in such transmission thus includes two longitudinally straight trajectory parts where it crosses from one pulley to the other and two longitudinally bent trajectory parts where it runs between and in frictional contact with the discs of a pulley at a respective radius of curvature for each of said two pulleys, which radii define the geometric transmission ratio of the transmission. In the said bent trajectory parts adjacent transverse elements mutually contact through their respective rocking edges.
The pulley discs exert a clamping force on the transverse elements of the drive belt, which force, on the one hand, enables torque transmission through friction between the respective pulley and the drive belt and, on the other hand, urges the transverse elements radially outward with respect to the axis of rotation of the respective pulley. The actual radial movement of the transverse elements is, however, limited by the carrier, so that (a radial component of) the clamping force is transmitted by the transverse elements to the carrier via a normal force there between. The carrier is thereby put under tension enabling torque transmission from a driving pulley to a driven pulley by means of the transverse elements pushing each other forward from the driving to the driven pulley along the circumference of the carrier, while being supported and guided thereby. On the other, so-called slack side of the drive belt, the transverse elements are returned from the driven pulley back to the driving pulley, however at a much lower, possibly even zero, pushing force existing there between.
It is common practice that the combined thickness of the transverse elements of the drive belt is slightly smaller than the circumference of the carrier, such that a gap exists in longitudinal direction between at least two adjacent, i.e. not physically abutting transverse elements, a/o for allowing the drive belt to be assembled without difficulty. This gap is denoted the belt's longitudinal play or clearance, which is defined and may be measured by placing the belt in a circular posture and sliding two adjacent transverse elements thereof apart, i.e. in mutually opposite longitudinal directions along the circumference of the carrier, until all of the transverse elements of the belt are mutually abutting through their respective rocking edges, thus forming a continuous array. The gap thus formed between the said two adjacent, but slid apart transverse elements defines the so-called initial static clearance that is to be measured at the radial position of the rocking edge. Hereby, it is remarked that during operation in the transmission the actual static or actual dynamic play in longitudinal direction, i.e. clearance, between the elements varies in dependency on the actual posture of the drive belt, i.e. on the geometric transmission ratio, as well as on the load applied thereto. Moreover, the belt's longitudinal clearance tends to increase over time, due to wear of the transverse elements during operation of the transmission. Indeed, it has been found by applicant that by the end of the service life of the drive belt, the ultimate dynamic clearance typically can amount up to ten times the said initial static clearance.
Although it greatly facilitates drive belt assembly, it has also been recognized in the art that longitudinal belt clearance has an adverse effect on transmission efficiency and/or wear, which may be understood as follows. When entering the driving pulley the transverse elements may have to overcome some resistance before they are taken up by the said pulley, in which case the elements have to be actively pushed between the discs the driving pulley to a certain extend, due to the interplay of forces in the transmission. To overcome this entry resistance at least some compressive force has to be build-up between the transverse elements in the slack straight return part of the drive belt's trajectory, whereby an array or string of mutually abutting elements will be formed in this trajectory part too. Moreover, once such transverse elements arrive into frictional contact with the driving pulley they are accelerated forward by the rotation thereof. By these two effects the dynamic clearance tends to be present at least in part in a first or entry part of the belt's longitudinally bent trajectory on the driving pulley. This means that only a part of this bent trajectory is available for the build-up of a pushing force between the elements, which latter, second part is denoted the active part of this bent trajectory wherein the elements are mutually abutting. In such active part the transverse elements are slipping backwards relative to the rotation of the driving pulley, whereby the clearance between adjacent elements present in the entry part is removed. However, by such slipping, wear occurs and energy is dissipated and transmission efficiency is adversely affected.